1. Field of the Invention
This invention relates to an anti-reflection film and more particularly, to an anti-reflection film for a plastic optical component to be used for an optical system such as projection television unit, video camera, still camera or the like.
2. Description of the Prior Art
Conventional optical component such as the lens or the like has employed inorganic glass as a rawmaterial in many cases. Recently, plastic materials are being increasingly introduced as a raw material for optical components due to their advantages that are light in weight, easy to manufacture and superior in mass- production performance. However, an optical component made of plastic material disadvantageously arises such a problem that surface reflection is so large as the inorganic glass does. In order to eliminate such disadvantage, the same material as the inorganic glass can be coated as an anti-reflection film on the surface of a plastic optical component. Anti-reflection film to be used for this purpose is generally formed by the vacuum evaporation technique
As an anti-reflection film using one kind of material to be evaporated, a single-layered anti-reflection film of magnesium fluoride (MgF.sub.2) is well-known. Anti-reflection film using SiO only is also in the prior art (see U.S. Pat. No. 4,497,539). As an anti-refection film using two kinds of materials to be evaporated, a three-layered anti-reflection film of silicon dioxide (SiO.sub.2) and magnesium fluoride (MgF.sub.2) and a two-layered anti-reflection film of cerium oxide (CeO.sub.2) and silicon oxide (SiO.sub.x) are disclosed in Japanese Patent Application Laid - Open No. 60-129701 and Japanese Patent Application Laid - Open No. 63-172201, respectively. In addition, as an anti-reflection film using three or more kinds of materials to be evaporated, a three-layered anti-reflection film of silicon dioxide (SiO.sub.2), aluminum oxide (AL.sub.2 O.sub.3) and cerium oxide (CeO.sub.2) has been proposed (see Japanese Patent Application Laid-Open No. 63-81402).
When a single-layered anti-reflection film is to be formed of magnesium fluoride by the vacuum evaporation technique, for example, due to such facts that plastic material is low in flow temperature and heat distortion temperature as well as allows to radiate a gas from the inside thereof, it cannot be formed as a hard anti-reflection film using such a board heating process that is carried out when an anti-reflection film is formed on the surface of an inorganic glass component (in which the heating temperature ranges from 300.degree. to 400.degree. C. in general). As a result, the formation of an anti-reflection film on the surface of a plastic optical component is carried out at a low temperature of 50.degree. to 60.degree. C. However, the anti-reflection film thus obtained is low in crystallinity of magnesium fluoride, which means that the obtained film is extremely vulnerably soft, resulting in reduction in durability. As a result, in order to improve the adhesion of an anti-reflection film and the surface of, a plastic optical plastic optical component is heated to a temperature of 60.degree. to 80 .degree. C. for vacuum evaporation or that the RF ion plating technique is used to form an anti-reflection film thereon. However, these methods require delicate controls of evaporation conditions, and easily cause cracks in the anti-reflection film during formation. It is difficult to constantly keep the forming conditions preventive from crack formation as well as to maintain the surface condition of a plastic optical component constant. Also, these methods are not suited to mass- production. In addition, plastic materials such as polymethyl methacrylate (PMMA) and diallyl glycol carbonate (CR39), which are frequently used as a raw material of optical components, are low in refractive index as about 1.49 and 1.50, respectively, so that in case of a single-layered anti-reflection film of magnesium fluoride (with a refractive index of about 1.38), a residual reflectance of about 1.5% exists, thus being unable to provide a suitable characteristic to the anti-reflection film.
Referring to the conventionally proposed three-layered anti-reflection film using silicon monoxide only (see U.S. Pat. No. 4,497,539), the silicon monoxide (SiO) as a material to be evaporated is well known to change the refractive index largely depending on evaporation conditions, ranging from about 1.50 to 1.90. The refractive index of SiO is subject to aging and as a result, there arises such a problem that the optical characteristic as an anti-reflection film will become unstable.
Referring to the three-layered anti-reflection film using magnesium fluoride and silicon dioxide (see Japanese Patent Application Laid-Open No.60-129701), a magnesium fluoride layer is sandwiched between two silicon oxide films, so that no formation of crack and comparatively good durability can be obtained. However, it has a residual reflectance as large as that of a single-layered anti-reflection film, being unable to provide a suitable characteristic thereto.
Further, referring to the two-layered anti-reflection film using cerium oxide and silicon oxide (see Japanese Patent Application Laid-Open No.63-172201), the refractive index of SiO.sub.x as a thin film material is subject to aging, so that there arises some problems on the stability of optical characteristics.
In addition, referring to the anti-reflection film using three or more kinds of materials to be evaporated (see Japanese Patent Application Laid-Open No. 63-81402), there requires material managements, resulting an increase in manufacturing cost.
As explained above, conventionally proposed anti-reflection films to be used for plastic optical components have been pointed out such that they are low in adhesion with a plastic surface, inferior in durability, unsatisfactory in optical characteristic as an anti-reflection film and unstable in the optical characteristic as well as not suited to mass- production.